1. Field of the Invention
This invention relates to a focus detecting apparatus for use in optical instruments such as cameras.
2. Description of the Prior Art
In optical instruments such as cameras, focus detecting apparatuses are well known in which the amount of relative deviation of a plurality of object images formed on the basis of lights passed through different pupil areas of an objective lens is detected by a sensor to thereby detect the focus state of the objective lens.
For example, an apparatus in which a fly-eye-lens is disposed in the predetermined imaging plane (a plane equivalent to the film surface) of the photo-taking lens of a camera and a sensor array is disposed rearwardly of the fly-eye-lens, whereby the amount of deviation of images corresponding to the focus state of the photo-taking lens is detected is disclosed in U.S. Pat. No. 4,185,191 (issued Jan. 22, 1980). Also, so-called secondary imaging type apparatuses in which a plurality of imaging lenses are juxtaposed rearwardly of the predetermined imaging plane of the photo-taking lens, whereby a plurality of object images are directed to a sensor array to thereby detect the amount of deviation of the images are disclosed in Japanese Laid-open Patent Application No. 118019/1980 (laid open on Sept. 10, 1980) and Japanese Laid-open Patent Application No. 155331/1980 (laid open on Dec. 3, 1980). The apparatus of this type has a more or less greater full length than the aforementioned apparatus, but it has a merit in that it does not require any special optical system such as a fly-eye-lens.
The principle of the secondary imaging type focus detecting apparatus will hereinafter be described briefly by reference to FIG. 1 of the accompanying drawings. A field lens 2 having the same optic axis as that of a photo-taking lens 1 whose focus is to be adjusted is disposed in the predetermined imaging plane of the photo-taking lens 1 and two secondary imaging lenses 3a and 3b are parallel-disposed rearwardly of the field lens 2, and light-receiving sensor arrays 4a and 4b each comprising a plurality of photoelectric conversion elements are further disposed rearwardly of the secondary imaging lenses. Reference characters 5a and 5b designate stops provided near the secondary imaging lenses 3a and 3b. The field lens 2 substantially images the exit pupil of the photo-taking lens 1 on the pupil planes of the two secondary imaging lenses 3a and 3b. As a result, light fluxes entering the secondary imaging lenses 3a and 3b emerge from the region of equal area on the exit pupil of the photo-taking lens 1 which correspond to the secondary imaging lenses 3a and 3b and which do not overlap each other. When the primary image O' of an object O formed near the field lens 2 by the photo-taking lens 1 is re-imaged as secondary images O" on the light-receiving surfaces of the sensor arrays 4a and 4b by the secondary imaging lenses 3a and 3b, the re-imaged two secondary images O" vary their positions on the basis of the difference between the positions in the direction of the optic axis at which the primary image O' is formed.
FIGS. 2A, 2B and 2C of the accompanying drawings illustrate the manner in which such phenomenon occurs. The two secondary images O" formed on the light-receiving surfaces of the sensor arrays 4a and 4b in the near-focus state and the far-focus state as shown in FIGS. 2B and 2C with the in-focus state of FIG. 2A as the center move in the opposite direction on the light-receiving surfaces of the sensor arrays 4a and 4b. If the then distributions of quantity of light of the secondary images O" are photoelectrically converted into electrical signals by the sensor arrays 4a and 4b and these signals are processed by an operating circuit to thereby detect the amount of relative positioned deviation of the two secondary images O", it will become possible to discriminate the focus state of the photo-taking lens 1.
As a method of processing the photoelectrically converted signals, there is known, for example, U.S. Pat. No. 4,250,376 (issued Feb. 10, 1981). According to this method, when the ith ones of data signals obtained by photoelectrically converting two secondary images O" by sensor arrays 4a and 4b each comprising N photoelectric conversion elements are a(i) and b(i) (i=1-N) and the amount of deviation of each image O" is V, ##EQU1## is calculated for a suitable constant k by an analog processing circuit or a digital processing circuit and the direction of movement of the photo-taking lens 1 for focusing is determined by the positive or the negative of the value of this amount of deviation V.
Also, in U.S. application Ser. No. 464,578 (filed Feb. 7, 1983), applicant has proposed a processing method in which ##EQU2## is calculated and the direction of movement of the photo-taking lens 1 is determined by the positive or the negative of the amount of deviation V. In these equations, min{x, y} represents the smaller one of two real numbers x and y, max{x, y} represents the greater one of two real numbers x and y, and k is a suitable constant which usually is 1.
However, in the operation process based on these equations (1)-(3), the defocused state of the photo-taking lens 1 is merely discriminated. So, in such a focus detecting apparatus, methods in which, with attention paid to the fact that the amount of deviation of two secondary images O" and the amount of defocus of the photo-taking lens 1 are in a proportional relation, an operation process comprising displacing one of the images relative to the other image is adopted to thereby detect the amount of defocus of the photo-taking lens 1 (the amount of movement of the photo-taking lens 1 to its in-focus position) are proposed, for example, by U.S. Pat. No. 4,333,007 (issued June 1, 1982) and U.S. Pat. No. 4,387,975 (issued June 14, 1983). For example, in U.S. Pat. No. 4,333,007, to find the amount of defocus of the photo-taking lens 1 from the amount of deviation of two images directed to the respective light-receiving surfaces of sensor arrays 4a and 4b, use is made of the data signals a(i) and b(i) from the sensor arrays 4a and 4b and an amount of displacement m(m is an integer value which satisfies m.sub.1 &lt;m.ltoreq.m.sub.2) for moving the image indicated by b(i) relative to the image indicated by a(i) in the operation process is set up and the amount of deviation V(m) for the amount of displacement m which is ##EQU3## is repetitively operated. The graph in which the value of the amount of deviation V(m) for the amount of displacement m is plotted is as shown in FIG. 3 of the accompanying drawings. Now, when the two images have become coincident with each other due to such relative movement of the images, the amount of deviation V(m) of equation (4) should be O and therefore, in the case of FIG. 3, there is present an amount of defocus of the order of 1.5 bits.
Also, even when equations (2) and (3) are used, the amount of defocus can be found by a similar process. For example, for the amount of displacement m, ##EQU4## may be repetitively operated to thereby find the zero-cross-point of the amount of deviation V(m).
Now, the operation for finding such an amount of defocus of the photo-taking lens 1 will require a remarkably long process time if the number of data signals a(i) and b(i), i.e., the number N of photoelectric conversion elements constituting each sensor array 4a, 4b, is great. For example, in equation (4), to find the amount of deviation V(m) for an amount of displacement m, there will be required 4N additions and subtractions, namely, 2N operations for finding the difference between the signs of the absolute values and 2N operations of the sum for finding the integrated value. On the other hand, the lower limit value m.sub.1 and the upper limit value m.sub.2 for determining the range of the amount of displacement m are generally m.sub.1 =(-N/2) and m.sub.2 =(N/2) and therefore, the number of the amounts of deviation V(m) to be calculated is of the order of N. Accordingly, where the amounts of deviation V(m) are to be calculated for the respective amounts of displacement m, there will be required about 4N.sup.2 operations. Actually, where such operations are to be carried out by a microcomputer provided within a camera, calculation steps such as updating of the memory address, discrimination of the operation area, etc. will also be necessary for each operation.
Therefore, if, in such an apparatus, the number of the elements of the sensor arrays 4a and 4b is increased to increase the number N of data signals a(i) and b(i) in order to improve the accuracy of the focus detecting function, the process time will become remarkably long to make it difficult to effect the focus detecting operation in real time because the entire amount of operation increases in proportion to N.sup.2. Accordingly, the conventional apparatus of such type has suffered from an inconvenience that either the accuracy of the focus detecting function or the speed of the focus detecting operation becomes insufficient.